1. Field of the Invention
The present invention relates to a power transmission system for a four-wheel drive vehicle, for distributing and transmitting a torque, as outputted from a power source through a speed change device, to front and rear wheels.
2. Related Art
The four-wheel drive vehicle is enabled to have an enhanced runability on a muddy road or a rough road by transmitting the driving force to all the front and rear wheels. Another advantage is that a running stability on the ordinary road is also enhanced by sharing the driving force among all the four front and rear wheels. Since the power transmission path is complicated, on the contrary, the power loss is increased by the friction. In order to prevent the braking phenomenon (i.e., the tight corner braking) at a turning time, on the other hand, there is needed a center differential for causing differential rotations between the front and rear wheels. Moreover, the provision of the center differential requires a limiting device for limiting the differential action of the center differential so as to prevent the so-called xe2x80x9coff-torquexe2x80x9d when any one of the front and rear wheels rotates idle due to a slip.
When the driving force outputted from the power source such as an engine is thus distributed and transmitted to the four front and rear wheels, various devices are necessary for eliminating the various disadvantages at the running time. When these devices are provided individually and randomly, however, the entire construction is enlarged to raise disadvantages that not only the mountability is degraded but also the weight of and the cost for the vehicle body are increased. Generally in the prior art, therefore, there have been tried technical improvements for using a device having a plurality of functions or for devising the array of various devices, as exemplified in Japanese Patent Laid-Open No. 9-123777.
In the system disclosed in this Laid-Open, a differential device (or a center differential) having a torque sensing type differential limiting function for limiting the differential torque between the front and rear wheels in accordance with the torque difference between the torque to be distributed to the front wheels and the torque to be distributed to the rear wheels is arranged coaxially with the front differential. Moreover, a direct coupling clutch (or a differential lock clutch) for integrating a member to transmit the torque to the front wheels and a member to transmit the torque to the rear wheels is also arranged coaxially with those center differential and front differential.
In the disclosed system, therefore, the torque inputted to the center differential is distributed and transmitted to the output member on the side of the front wheels and the output member on the side of the rear wheels when the direct coupling clutch is released. When a speed difference occurs between the front wheels and the rear wheels at a turning time, moreover, the center differential performs the differential action to absorb the speed difference. When the torque difference increases, moreover, the differential limiting torque rises to limit the differential rotations between the front and rear wheels.
When the direct coupling clutch is applied, on the contrary, the two output members in the center differential are integrally coupled by the direct coupling clutch so that the center differential rotates as a whole to eliminate the differential rotations between the front and rear wheels. In other words, the torque can be transmitted, even if any one of the front and rear wheels rotates idly, to the remaining wheels.
In the system disclosed in the Laid-Open, the means for the differential limit of the front and rear wheels need not be provided separately from the center differential, but the center differential, the front differential and the direct coupling clutch can be arrayed on the common line so that the system can be made compact as a whole.
When the coefficient of friction on the road surface is low as on a muddy road or a seriously undulating rough road or when the individual wheels have different coefficients of friction with the road surface, the run can be ensured by distributing the torque among the four front and rear wheels while performing the differential limit, as described above. However, even the four-wheel drive vehicle does not run exclusively on the rough road or a road having low friction coefficient but frequently runs on an excellent car road paved with asphalt to have a high friction coefficient. If the driving force is transmitted to all the four front and rear wheels even on the excellent car road, an unnecessary power loss may be caused by the friction at the power transmission system.
The four-wheel drive system of the prior art thus far described is enabled to perform the differential action by setting the center differential free or not by locking the center differential by the direct coupling clutch. With or without the differential rotations between the front and rear wheels, however, what is established is the four-wheel drive state in which the power is transmitted to the front and rear wheels, but cannot be the two-wheel drive state in which the power is transmitted to either the front wheels or the rear wheels. In the case of running on a dry paved road, therefore, the fuel economy may be deteriorated by the friction loss. In other words, the two-wheel drive state cannot be set to invite a disadvantage in the versatility.
In the prior art, on the other hand, there is known the so-called xe2x80x9cpart-timexe2x80x9d four-wheel drive system capable of selecting the two-wheel drive state and the four-wheel drive state by the manual operation. This system is provided with a switching mechanism for coupling a transfer to transmit the power to the front wheels or the rear wheels selectively to the output member of a transmission. In this four-wheel drive system, the four-wheel drive state is established by coupling the transfer and the output member of the transmission, and the power is transmitted to only the front wheels or the rear wheels to establish the two-wheel drive state by releasing the coupled state. The switching mechanism of this kind can be added to the aforementioned system disclosed in the Laid-Open. In this case, the system may be large-sized or the characteristics of the individual drive states may change, depending upon the arrangement or assembly of the switching mechanism, so that the technical items to be considered diverge in many ways. In the prior art, however, the space efficiency or the mountability has neither been considered, nor has developed any proper array for the switching mechanism or the transfer.
Moreover, the four-wheel drive vehicle is intrinsically intended to improve the runability on a rough road or the uphill performance. In addition of the ordinary transmission for setting a gear ratio according to the vehicle speed or the accelerator opening, therefore, a speed change mechanism for switching the high-low gear ratios may be provided for especially raising the driving force. This speed change mechanism is associated with the four-wheel drive system such as the transfer or differential mechanism so that it is arranged together with the latter. In the prior art, however, neither special investigation nor technical development has been made on the arrangement of the speed change mechanism for switching the high-low gear ratios, but it is customary merely to array the speed change mechanism on the axis common to the differential mechanism or the like. This may elongate the four-wheel drive system in its entirety and deteriorate its mountability.
A main object of the invention is to provide a power transmission system for a four-wheel drive vehicle, which can be made compact.
Another object of the invention is to provide a power transmission system for a four-wheel drive vehicle, which is enabled to reduce the size by reducing the external diameter of a differential mechanism to make effective use of a surplus space around the differential mechanism.
Still another object of the invention is to provide a power transmission system which can set a two-wheel drive state and four-wheel drive states with and without a differential action and which is short in its overall length.
A further object of the invention is to provide a power transmission system for a four-wheel drive vehicle, which can switch an input to a differential mechanism for differential rotations between the front wheels and the rear wheels and which can be shortened in its overall length.
According to an aspect of the invention, therefore, there is provided a power transmission system for a four-wheel drive vehicle, comprising: a differential mechanism having a torque sensing type limiting function and including an input element and two output elements made rotatable differentially of each other by receiving a power from the input element for establishing a torque limiting a differential rotation in response to the input torque, one of the two output elements being integrated with a front wheel side drive member, and the other output element being coupled to a rear wheel side drive member. Moreover, this power transmission system further comprises: a torque distribution mechanism for transmitting the torque to a front-wheel drive shaft connected to the front wheels or a rear-wheel drive shaft connected to the rear wheels; a first selective coupling mechanism for coupling the torque distribution mechanism selectively to the front-wheel side drive member or the rear-wheel side drive member; and a second selective coupling mechanism for coupling the input member and one of the output members, when the first selective coupling mechanism releases the coupling between the torque distributing mechanism and the front-wheel or rear-wheel side drive member, to integrate the differential mechanism thereby to establish a two-wheel drive state.
In the power transmission system of the invention, therefore, the torque is inputted to the input member in the differential mechanism and is transmitted to the individual output members and further to the drive member on the side of the front wheels and the drive member on the side of the rear wheels. When the front-wheel side drive member or the rear-wheel side drive member and the torque distributing mechanism are coupled in this case by the first selective coupling mechanism, the torque transmitted to the input member is distributed and transmitted to the front wheels and the rear wheels to establish the four-wheel drive state. On the other hand, the differential mechanism comes into the so-called xe2x80x9cfree statexe2x80x9d so that the differential limiting force rises according to the input torque thereby to execute the torque sensing type differential limit. On the other hand, the differential mechanism can be integrated as a whole by the second selective coupling mechanism. In this state, the torque distributing mechanism can be decoupled from the front-wheel side drive member or the rear-wheel side drive member by the first selective coupling mechanism thereby to establish the two-wheel drive state in which the differential mechanism is integrated, so that the torque inputted to the differential mechanism is outputted as it is from any output member to the drive member on the front or rear wheel side. In short, the four-wheel drive and the two-wheel drive can be switched.
On the other hand, the differential mechanism having the torque sensing type differential limiting function in the invention can be exemplified by a construction in which the planet pinion gear is held in frictional contact with the inside of the case member. By coupling one side gear meshing with the planet pinion gear and the case member by the coupling means, moreover, the differential mechanism is integrated to inhibit the differential action. Simultaneously with this, the coupling means is decoupled from the means for transmitting the torque to the rear wheels, thus establishing the two-wheel drive state.
In the invention, moreover, the first selective coupling mechanism includes: a mechanism for coupling the one side gear and the torque distributing mechanism when the case member and the one side gear are coupled by the second selective coupling mechanism; and a mechanism for coupling the one side gear and the torque distributing mechanism when the case member and the one side gear are decoupled by the second selective coupling mechanism.
With this construction, it is possible to set: the two-wheel drive state in which the transmission of the torque to the torque distributing mechanism is blocked when the case member and the one side gear are coupled by the second selective coupling mechanism to integrate the differential mechanism as a whole; the so-called xe2x80x9cdirect coupledxe2x80x9d four-wheel drive state, in which the torque is transmitted to the torque distributing mechanism by the first selective coupling mechanism when the differential mechanism is integrated as a whole, that is, the four-wheel drive state in which the differential rotations between the front and rear wheels are not effected; and the four-wheel drive state in which the case member and the one side gear are decoupled by the second selective coupling mechanism to set the differential mechanism into the so-called xe2x80x9cfree statexe2x80x9d and in which the torque is transmitted to the torque distributing mechanism by the first selective coupling mechanism, that is, the four-wheel drive state in which the differential rotations between the front and rear wheels are allowed by the differential mechanism.
In the power transmission system of the invention, still moreover, there are arrayed on a straight line: a first position at which the first selective coupling mechanism is set to decouple the one side gear and the torque distributing mechanism and at which the second selective coupling mechanism is set to couple the one side gear and the case member; a second position at which the first selective coupling mechanism is set to couple the one side gear and the torque distributing mechanism and at which the second selective coupling mechanism is set to couple the one side gear and the case member; and a third position at which the first selective coupling mechanism is set to couple the one side gear and the torque distributing mechanism and at which the second selective coupling mechanism is set to decouple the one side gear and the case member. The power transmission system further comprises a switching mechanism adapted to be linearly moved to and positioned at any of the first position to the third position for switching the individual selective coupling mechanisms.
With this construction, the switching mechanism linearly acts to move to the first to third positions so that the two-wheel drive state, the directly coupled four-wheel drive state and the four-wheel drive state allowing the differential rotations between the front and rear wheels.
In the invention, the differential mechanism can be constructed such that the planet pinion gear meshing with the paired side gears is held in sliding contact inside of the case member. Moreover, any two of the paired side gears and the case member output the torque to the front wheels and the rear wheels, and it is possible to provide a speed change mechanism around the other member for switching the high-low gear ratios. On the other hand, the other member may be the case member.
With this construction, the differential mechanism and the speed change mechanism are juxtaposed not in the axial direction but in the radial direction. With the differential mechanism being constructed so that the pinion gears meshing with the paired side gears are held in the case member, therefore, it is possible to effectively utilize the surplus space which is formed around the outer circumference in the radial direction. As a result, the power transmission system can be shortened in its entire axial length.
Here, the differential mechanism in the invention may be constructed such that the planet pinion gear held by the case member meshes simultaneously with both the paired side gears arranged on the common axis. Alternatively, the construction may be modified so that one of the paired planet pinion gears meshing with each other is meshed by one side gear whereas the other planet pinion gear is meshed by the other side gear.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.